What incident in 1960 heightened cold war tensions between the u. s. and the soviet union?

Eisenhower planned that the US and the USSR be able to fly over each other's territory to safeguard against unexpected nuclear attacks. But USSR said no. This led  CIA to start a secret high-altitude spy flights over Soviet territory called U-2s. In May 1960, USSR shot down a U-2 plane and its pilot was captured which heightened the tension.